Analyzing method and apparatus



3 Sheets-Sme t 1 D. P. ECKMAN ET AL ANALYZING METHOD AND APPARATUS FIG.I

May 17, 1949.

Filed March 27, 1945 INVENTOR. DONALD P ECKMAN May 17, 1949.

Filed March 2'7, 1945 FINAL VALUE OF VARIABLE D. P. ECKMAN ET AL2,470,434

ANALYZING METHOD AND APPARATUS 5 Sheets-Sheet 2 F I G. 3 FIG. 5 I

2 a g' 1 LU O u: a: c: U1 U1 3 a n: o 2 I00 0: 5 u: D m D D. 2 g S E 0 2o -2 -a 0 -5 -|o VOLTS INPUT (0c) F I G 4 I PU VOLTS \LAG FOR RATE OFoNE PER SECOND 1 CURRENT INTO PROCESS .UJ I

O l REFERENCE 0 T z PEN MOVEMENT I l 50 I00 I50 TIME IN 5935. FIG. 6

RANGE FACTOR =4.o

46 I4 60 K/v- I 47 T Y 40 I 1M 20 5 l0 l5 2o TIME-MIN. INVENTOR.

DONALD F. ECKMAN BY WILLIAM H.WANNAMAKER JR.

ATTORNE:

3 Shets-Sheet 5 INVENTOR. DONALD P ECKMAN ATTORN May 17, 1949. D. P.ECKMAN ET AL ANALYZING METHOD AND APPARATUS Filed March 27, 1945 N. N. mm M ME ME MU Mm T L L R R .3 m aw onw O/ODIV O/OPV O 5 n- 8 4 2 fimw Q34. 3% 25 DE D DE DE DE N v NT NT NT A A A A B BR BR BR BR R T T .T RURUN PE PE W u am mmm mmm mmm REP MRS PRS PRS PR8 PRS H i I I H! In a m I.FIG. 7

WILUAM H.WANNAMAKER JR.

Patented May 17, 1949 ANALYZING METHOD AND APPARATUS Donald P. Eckman,Philadelphia, and William H. Wannamaker, Jr., Flourtown, Pa., assignors,by mesne assignments, to Minneapolis-Honeywell Regulator Company,Minneapolis, Minn, a corporation of Delaware Application March 27, 1945,Serial No. 585,124

21 Claims. 1

The present invention relates to a method of and apparatus for analyzingand studying automatic regulation and control procedures, and, moreespecially, pertains to a method of and apparatus for artificiallycreating or duplicating characteristics of various physical processes inorder to facilitate investigation of the effect of said characteristicsand/or the effect of inherent qualities of the automatic controlapparatus on the automatic regulation or control obtained.

in fundamental investigations of automatic control procedures, a needoften arises for process simulation or duplication. Automatic controlprocedures may be studied in the field on actual control applications,but such investigations are ordinarily very difficult to carry onbecause the process under investigation is subject to changingconditions over which there is little or no control. Moreover, in actualcontrol applications in the field, the introduction of intentional loadchanges, disturbances or control point shifts, which it is necessary tomake in order to determine the dynamic action of the control system, isgenerally not permissible. As a consequence, various methods andapparatus have heretofore been proposed for simulating in the laboratoryprocess characteristics for the purpose of facilitating theinvestigation of automatic control procedures and apparatus.

The method and apparatus of the present invention offer advantages ofversatility which render the invention especially suitable and desirablefor accomplishing such investigations in the laboratory, Specifically,the dynamic action of the control system applied to control or regulatethe simulated process may be readily and accurately observed, and anyfactor, either in the simulated process or in the automatic controller,may be easily segregated and altered.

An object of the invention is to provide an improved method of anapparatus for artificially creating or duplicating characteristics ofphysical processes. It is a more specific object to attain this resultby means of an electrical analogue utilizing electrical resistance andcapacitance circuits.

Another object of the invention is to provide in conjunction withindicating type automatic control apparatus an improved method of andapparatus for studying the effect on the indicating means, and therebyon the characteristic under investigation, of variations in such factorsas throttling range and/r reset rate.

A further object of the invention is to provide an improved method ofand apparatus for studying the eifect on the indicating means, andthereby on the characteristic being investigated, 01 such changes in aprocess, or in various difiere'nt processes, as shifting of the controlpoint, load changes, control agent supply changes, and/or changes inso-called dead time. Dead time may be defined as the elapsed timebetween the occurrence of a change in a factor operative to ultimatelycause a change in the measured characteristic, and the time when suchchange in the measured characteristic actually takes place. A furtherand specific object of the invention is to provide a simple andefficient electrical circuit arrangement for introducing such dead timeinto the artificially simulated process operation.

A still further object of the invention is to provide a method of andapparatus for investigating the effect on the indicating means, andthereby on the characteristic under investigation, of changes in thecontroller dead zone, changes in the characteristics of the controlvalve employed, and also changes in any factor involving any operationof the complete system under investigation. It is also a specific objectto provide a simple and efficient electrical circuit arrangement forartificially simulating characteristics of a control valve and forreadily adjusting the size and range of the control valve.

It is a further object of the invention to provide an improved method ofand apparatus for ascertaining the operating characteristics andrelative merits of different types of automatic controllers applied tosimilar processes.

The various features of novelty which characterize our invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,however, its advantages, and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which we have illustrated and described preferred embodimentsof the invention.

Of the drawings:

Fig. l is a schematic illustration of a preferred form of apparatuswhich may be utilized to carry out the method of the present invention;

Fig. 2 shows in more or less diagrammatic manner, the details of thevarious units shown schematically in Fig. 1;

Fig. 3 is a graph showing the output current versus applied inputvoltage relation of the current input unit of Fig. 2;

Fig. i is a graph illustrating the effect of the dead time unit of Figs.1 and 2;

Fig. illustrates calibration curves obtainable with the vacuum tubevoltmeter of Figs. 1 and 2;

Fig. 6 is a, graph showing a reaction curve of a process simulated bymeans of the arrangement of Figs. land 2; and

Fig. 7 is' a graph showing a number of recovery curves for the processhaving the reaction curve of Fig. 6.

Prior workers in the art have proposed techniques of applying control tosimulated processes.

The techniques proposed, however, were limited in their application inthat they did not lend themselves readily to adjustment for duplicatinga variety of multiple capacity processes.- M'ul tiple capacity processesare those possessing two or more capacities each separated from eachother by resistance to flow of energy between them. The electricalanalogue of the present invention, on the other hand, possesses extremefl'eiiibility for duplicating or simulating numerous c's'se'smereiy byaltering the values of resists and condensers, which alterations may bee sily" accomplished in practice, as by means of pure and jackconnections.

Inasmuch as a direct relationship has been feurid toexist betweenelectrical units and thermal and hydraulic units, the physical constantsof th'e're'sistances and condensers utilized may be calculated usingwell known theory. The relationship between electrical, thermal andhydraulic units is illustrated by means of the followir'igtabl'; In thetable the units are based on po'int values in order to avoid thecomplexity of power-functions. The tim basis of the analogy may bear anychosen convenient ratio to that of the actual process although analtered time relation may introduce complications when the controlsysteminvolves integral and derivative functions.

Most industrial controllers are provided with some form of pneumaticunit, electrical contact means,- or power-set slider on a resistanceslidewire for accomplishing the desired control actions on the processto which the controller is applied. The method and apparatus of thepresent invention are readily adaptable for use with any one ofsuchindustrial controllers, but for purposes of illustration we haveshown in Fig. 1 the controller as comprising a self balancingpotentiometer I equipped with pneumatic control means supplied with airfrom a suitable source through apipe Zior causing the air pressure to adiaphragm valve motor 3 toposition a sliding contact 4 along a slidewireresistance 5. The potentiometer I and the pneumatic control meansincluding the diaphragm motor 3 may be of the type commerciallymanufactured and sold by The Brown-Instrument Company and respectivelydisclosed in the Wills application Serial Number $21 173, filedDecember1, 1941, now Patent No. 2,423,540 of July 8, 1947, and in the MoorePatent 2,125,081, granted July 26, 1938.

A battery 6 and an adjustable resistance 1 are connected across theslidewire resistance 5 for producing a voltage drop of known magnitudeacross the later. A portion of this voltage drop is tapped off theslidewire 5 by the sliding contact 4 for deriving a voltage of variablemagnitude which is impressed on the terminals 8 of a unit 9 which I havefor convenience termed a dead time unit. The output terminals I0 of thedead time unit 9 are connected to the input terminals II of a unit I2,conveniently termed a current input unit, having output terminals I3which are connected to the input terminals I4 of a unit designed tosimulate the capacity and transfer lags of a process under investigationand comprising a resistance-capacitance network I5.

The values of resistance and capacitance in the network I 5 in theclosed circuit control system of Fig. 1 determine the characteristics ofthe simulated process under control. The various values of thecondensers and resistances are so chosen as to introduce a delay in thevoltage transfer through the network from the input terminals I4 to theoutput terminals I6 corresponding to the delay (resulting from thcapacity and transfer lags of the process) occurring between the time achange in the supply of a controlling agent to an actual process undercontrol is made and the time when the process has attained a stabilizedcondition with the new supply of controlling agent.

The output terminals I6 of the network I5 are connected to the inputterminals I! of an electronic voltmeter I8 having output terminals I9which are connected to the input terminals of the potentiometercontroller I.

As may be seen by reference to Fig. 2, which illustrates in more detailthe various units of the arrangement of Fig. 1, the dead time unit 9in'- cludes a number of condensers 20, which preferably are high gradecondensers, and a ratchet type multiple contact switch indicatedgenerally at 21, and comprising two sections, on provided with arotatable contact arm 22 and the other provided with a rotatable contactarm 23. While only twelve condensers and twelve contacts have been shownin connection with each section of the multiple contact switch 2|, thenumber of condensers and the number of points on each section of themultiple contact switch may be larger or smaller as desired. A physicalembodiment of the arrangement shown actually comprised twenty-fivecondensers and twenty-five points'on each section of the multiplecontact switch. The ratchet type multiple contact switch 2I may be ofany suitable type and, for example, may be of the well known telephonetype.

An electronic pulse circuit, indicated generally at 24, and anassociated rel'ay'25 are provided for actuating the multiple contactswitch II to ad- Vance the contact blades 22 and 23, one step at'a time,from one point to the next succeeding point.

One input terminal 8 of the dead time unit-9 is connected to a commonterminal of each of the condensers 2!], and the other input terminal 8is connected to the rotatable contact arm 22. The other terminal of eachof the condensers 20 is connected to an individual one of the points onthe contact switch section associated with the rotatable switch arm 22.Consequently, as the arm 22 is advanced, step by step, from one contactpoint to the next, an electrical charge is applied to each of theconden'sers ZlJ in succession from the slidewire resistor 5. Themagnitude of the charge applied to the condensers is directlydeterlength of the slidewire resistance 5.

As shown in Fig. 2, each of the second mentioned terminals of thecondensers is also connected to an individual one of the contact pointson the contact switch section which is associated with the rotatablecontact arm 23. The switch arm 23 is connected to one output terminal IDof the dead time unit and thereby to one input terminal ll of thecurrent input unit l2, and the other input terminal ll of the latter isconnected to each of the first mentioned terminals of the condensers 29.The two switch contact arms 22 and 23 are out of step by one positionand the wiring is such that each previously charged condenser appliesits potential to the input terminals ll of the current input unit l2 fora period equal to the time between steps. Specifically, the switch arm23 is shown as being ahead of the switch arm 22 by one step so that thedead time unit 9 effectively produces a dead time equal to eleven timesthe stepping or pulsing period in the arrangement shown. If desired,

the switch arm 22 may be made to move in step with switch arm 23 and thewiring from the condensers 29 to the contact points of the switch 2|arranged so that the switch arm 23 will be electrically ahead of the arm22 by one step. When a larger number of condensers is utilized, thestepping or pulsing period remaining the same, the dead time iscorrespondingly larger. For example, when twenty-five condensers areemployed, the dead time is equal to twenty-four times the pulsingperiod. It is noted that the dead time may be increased or decreased induration by either varying the number of condensers and contact pointson the switch sections, while maintaining the pulsing period constant,or by varying the duration of the pulsing period. The latter method ispreferred because variation in the dead time is easier of accomplishmentby this method.

The adjustable electronic pulse circuit 24 is supplied with energizingcurrent from a source of alternating current of commercial frequencywhich is adapted to be connected to the terminals 26. The circuit 24includes two electronic tubes designated by the numerals 21 and 28which, as shown, are heater type triodes. Energizing current is suppliedto the filaments of each of the triodes 21 and 28 from the secondarywinding of a transformer 29 having its primary winding connected acrossthe terminals 26. A potentiometer resistance 30, provided with a slidingcontact 3!, is also connected across the terminals 26, the cathode ofthe triode 21 being connected to the sliding contact 3!. The controlgrid of the triode 21 is connected through a suitable resistance to thelower terminal of resistance 30, as seen in the drawing. Accordingly,the voltage drop appearing between the contact 3| and the lower terminalof resistance 30 provides suitable bias voltage for the control grid ofthe triode 21. The magnitude of this bias voltage may be adjusted asdesired by sliding the contact 3| along the length of resistance 30. Theanode of the triode 21 is connected by a condenser 32 to the upperterminal of the resistance 39. Thus, alternating voltage of commercialfrequency is supplied to the anode circuit of the triode 21, whereby thetriode 21 is periodically rendered conductive at the frequency of thesupply source.

The output circuit of the triode 28 is connected to the terminals 26 insuch manner that this triode is adapted to be rendered conductive duringhalf cycles which alternate with those during which the triode 21 isadapted to be rendered conductive. The output circuit of triode 28includes the operating coil 33 of a relay 34 which is adapted to close aswitch 35 when the relay is energized. The switch 35 is opened when therelay is deenergized, being biased by gravity or other suitable means toits open position. A condenser 36 is connected in shunt to the operatingcoil 33 for smoothing out the current flow through the latter and,thereby, for eliminating relay chatter. The input circuit of the triode28 is controlled in accordance with the voltage produced across thecondenser 32 which is included in the output circuit of the triode 21.

When the operation of the apparatus is initiated, the potential dropacross the condenser 32 is zero, and consequently, the control grid ofthe triode 28 is then at the same potential as its associated cathode.The triode 28 is then fully conductive and energizes the relay 34 toclose the switch 35. This action serves to close an energizing circuitto the operating coil 31 of the relay 25 from a battery 38. As a result,the relay 25 then advances the contact arms 22 and 23 of the switchingmechanism 2| to the next position.

As the triode 21 is periodically rendered conductive during alternatehalf cycles of the commercial alternating current supply source, acharge is gradually built up on the condenser 32 in the direction torender the control grid of the triode 28 negative in potential withrespect to the potential of its associated cathode. This actioncontinues until the conduction of the triode 28 is reduced to such anextent that the relay 34 is deenergized. Upon such occurrence, theswitch 35 is actuated to its open position whereupon relay 25 isdeenergized. Relay 25, as shown, is also provided with a switch 39which, when closed, short circuits the terminals of the condenser 32 andconsequently discharges the latter. Switch 39 is actuated by gravity orother suitable means to its closed position when relay 25 is deenergizedand is moved into its open position when relay 25 is energized.Accordingly, when triode 28 is rendered non-conductive due to the chargestored by condenser 32, the deenergizing action of relay 25 operates topermit switch 39 to close and thereby short-circuit condenser 32. Thisdischarges condenser 32, and hence, permits triode 28 to again conductand energize relay 34. Such energization of relay 34 operates to causeenergization of relay 25, which in turn advances the rotatable switcharms 22 and 23 to their next position.

As those skilled in the art will understand, the duration of the periodsduring which triode 28 is surficiently conductive to energize relay 34may be readily controlled by adjustment of the sliding contact 3i alongthe length of the resistance 39. For example, by increasing the negativebias applied to the control grid of the triode 21, the time required tocharge condenser 32 sufficiently negative to render the triode 28non-conductive is correspondingly increased, and vice versa.

In accordance with the present invention, the sliding contact 3i isadjusted to a position along the length of the resistance 30corresponding to the magnitude of the dead time it is desired tointroduce into the simulated process. In this manner, a readilyadjustable means is provided for varying the time interval between theapplication of a change in the voltage applied to the input terminals ofthe dead time unit and the time 7 when thatvoltage changeappears betweenthe output' terminals of 'thedead time unit. This delayin'the appearanceoirth voltage change between the input and output terminals of the deadtime'unit corresponds to the delay occurring in actualicompletccontrolsystems between the time of: application of a change at some point inthe system, which is' operative to influence a factor of the processunder control, and the time when suchchange has reacted through thecomplete system to-cause a change in the opposite sense at -the pointwhere-the original change was initiated:

The current input unit I2 is employed to provide a source of currentin-its'output circuit which is regulated in value by adjustment of thevoltage applied to'its input terminals I I and the range of valuesofwhichwmay also be varied as desired. A-characteristic of the inputcurrent unit is that itmayxbe' utilized to simulate a mechanical valve,for example, regulating th flow of fuel to a-iurnace, the valve opening.being adjusted in accordance with the voltage impressed on theinputterminals ll. Meansto be described are also included-in the inputcurrent unit for varying the size of-the simulated valve.

Specifica1ly,.the. input current unit 52 is provided with two heatertype triodes respectively indicated by the reference numerals 4B and 4|,a battery 42 forsupplying anodeor output current tobothtriodes inseries, a-battery 43 for supplyinglsuitable bias voltage to the inputcircuit of the triode llL-and. anadjustable resistance 44' for supplyingsuitable bias voltage to the input circuit of the triode 40. It will beunderstood that suitable regulatedvoltage supplies energized from acommercial alternating current source maybe employed, instead. ofbatteries 42 and 43, if desired. A condenser 45, which preferably is ofsmall value, is: connected across the input terminals II for smoothingout the voltage pulsations-applied to the input terminals of the inputcurrent unit [2 from the output terminals of the dead time unit 9 and tostabilize the potential of the control grid of tri'ode 40 during theswitching intervals.

The operation of the current input unit [2 in maintaining asubstantially constant flow of current in its output circuit,notwithstanding changes in the voltages in the battery dzand changes inthe magnitude of the load impedance between the terminals [3' (offeredby the network f)", may 'bemathematically explained in the followingmanner.

Tracing through the complete circuit having connected therein theimpedance network f5 and the anode-cathode circuits of the triodes 4tand 41, Equati'on1' may be obtained by application of mi'ehhoffsprinciple that the sum of the voltage drops in a closed circuit must beequal to zero;-

Where Ris 'is the l'oadresist'ance presented by the impedance networkvI5 between" the output. terminals I3 of the current input unit I22"; Iis the output currentfiow from the current input unit. l2 througntheimpedance network 5.5;?1344 istheresistance oi. thecathodebia's-iresistor M; moandun are the amplification factors oftheitriodes 40 and 41 respectively; R40 and R41 are: the anode tocathode 'resistances of; tubes till and M respectively; Eu isthe signalvoltage impressed. across: the in- '8 put-terminals H of thecurrentinput unit I! from the slidewire resistance-'5; E43 is thevoltage of battery 43-; audio is a constant.

Differentiating Equation. 1 with respect to R115, Equations 2, 3 and 4are obtained;

Equation 4 is'obtained by transposing and by substituting P for As thoseskilled'in the art will'understand -the quantity T11 represents theratio of the current change in the output circuit of the current inputunit ['2 for a given change in impedance between the output terminalsl3. By inspection of Equation 4 it may be observed that the current 11)flowing in the output-circuit of the current input unit I2 issubstantially independent of the magnitude of the load impedance R15over a range extending from" zero up to one megohm and higher values ofresistance.

Merely by way of illustration,v and not limitation, it is noted that,when triodes 40 and 4f are of the commercially'available 'IFT type, theother circuit components of the current input unit may have thefollowing values:

Part Value 5,000150,000 ohms.

300' volts.

T volts.

0.05 microfarads.

When these circuit components are empolyed and resistances R15 and. R44,for example,v are assigned-values: of 500,000 ohms and) 100,000 ohmsrespectively, the value of the quantity may be readily calculated andfound to be approximately 01001 when the anode to cathode resistances ofeachof the triodes 40' and 4| is assumed to be 20,000 ohms and. theamplification factor u'of each of the triodes is assumed to be 70.Accordingly, the changetin'v output current fiow'from the current inputunit l2 upon change in the magnitude of the load resistance Riswill beor, in -other words, only 0.1% of the ratio I the change in" loadresistance Brads or 0.05%. Consequently, there is an insignificantchange in the flow of current from the output circuit of the inputcurrent unit l2 upon change in the value of the load resistance R15 overan extended range.

Fig. 3 is graph illustrating curves of anode or output current flow inthe ouput circuits of the triodes 40 and 4| plotted against voltageimpressed on the input terminals H from the dead time unit 9 for variousvalues of resistance 44. In this graph it is contemplated that the loadimpedance connected in the output circuit of the triodes 40 and 4| mayhave any value from zero to at least one megohm and to higher values, ifdesired.

The output current flow derived from the input current unit 12 isimpressed upon the input terminals 14 of the resistance-capacitance net-Work l constituting the simulated process. The value of the current flowto the resistancecapacitance network I5 is a function of and isdetermined by the voltage impressed on the input terminals H of theinput current unit I2 from the dead time unit 9. The output voltagevariations which appear between the output terminals N5 of theresistance-capacitance network l5 simulate the variable being measuredfor an actual process, such, for example, as the temperature within afurnace. In order to measure the voltage variations between theterminals I6, the vacuum tube voltmeter I8 is employed in conjunctionwith the potentiometer controller I. The vacuum tube voltmeter i8 servesas an impedance matching device to permit the proper functioning of thepotentiometer controller I, which controllers ordinarily are designed towork from low voltage, low impedance circuits.

The resistance-capacitance network I5 includes a number of resistances40 and also a number of parallel connected resistance 41 and condensers48. The condensers 48 may be of any suitable type and, for example, maybe paper type condensers or may be dry electrolytic condensers. In theaforementioned practical working embodiment of the invention, thecondensers 48 are wired to colored jacks on a Bakelite panel, and about30,000 microfarads of capacity are used together with about thirtyadjustable resistors 46 and 41. These are adapted to be connected bymeans of jumpers into jacks to form various types of mesh circuits. Abuilt-in bridge circuit and null indicator (both not shown) are alsoincluded for circuit measuring purposes. It is noted that when thecondensers 48 are of the electrolytic type, a circuit calling for highershunt resistances than are inherent in the condensers, in the form ofleakage, may not be utilized. In addition, variation of capacity andleakage, both with ambient temperature and impressed voltage, must betaken into account. While electrolytic condensers impose suchlimitations upon the use of the apparatus, a real advantage is gained intheir use, however, in that many industrial process applications may besimulated satisfactorily with considerable saving in cost and space.

The vacuum tube voltmeter l8 comprises av sharp cut-off heater typetube, preferably a pentode but shown for convenience as a triode 49,which is operated at a low plate voltage. A large series grid resistor50, connected between the control grid of triode 49 and one outputterminal l6 of the resistance-capacitance network I5, is utilized toreduce grid current to a low value. Anode current is supplied to thetriode 49 from a battery 5! through an adjustable resistance 52 andfixed resistances 54 and 55. The potential drop across an adjustableportion of the load resistance 52 provides the voltage to operate thepotentiometer controller I. In order to make the potentiometercontroller read upscale for an increasingly negative voltage applied tothe control grid of the triode 49, an adjustable resistance 53 isprovided in shunt to the battery 5| to derive a voltage which may beopposed to the voltage drop across resistance 52. That is to say, thevoltage drops across the resistors 52 and 53 are equal, whereby theirresultant is zero, when zero voltage is impressed on the control grid ofthe triode 40 from resist-. ance-capacitance network [5.

Preferably, the anode voltage supply derived from the battery 5! and thesource of voltage for energizing the heater filament of triode 49 areregulated. Such regulating means are desirable especially when thesevoltages are derived from an alternating current source. Desirably, twovacuum tube voltmeters may be provided so that one may be employed toactuate the potentiometer controller, as shown, and the other may beemployed to actuate a recorder to show the demand or the actual fuelflowing, or may be utilized to indicate the voltage Within any portionof the process mesh circuit 15.

In the aforementioned practical working embodiment of the presentinvention, an impedance measuring bridge is built into the panel board,with jacks for inserting leads from the various circuit components, andan ordinary electric amplifier and electron-ray tube of the indicatingtype are utilized as a null indicator.

In the operation of the arrangement shown in Fig. 2, the diaphragm motorpositions the sliding contact 4 along the length of the slidewireresistance 5 in accordance with the variations in the applied airpressure and thereby, according to the variations in magnitude of thevoltage impresed on the input terminals of the potentiometer controllerI. Since the number of valve positions is limited to the number ofconvolutions provided on the slidewire resistance 5, this number is madesuitably high and, for example, may be 200, although more convolutionsor fewer convolutions may be employed. A greater number of convolutionsthan 200 may require the use of a valve positioner. The sensitivityobtained with 200- convolutions has proved satisfactory when used inconjunction with the dead time unit 9 with its step function, asdescribed.

It is noted that in low capacity processes where low rates of switchingwould be objectionable, dead time to any large degree is generally notmet in actual practice and the consequent fast pulsing of the dead timeunit 9 provides a satisfactory dead time even for such low capacityprocesses. The twenty-five position selector switch iii was chosen inthe aforementioned representative embodiment of the invention as themost feasible device to approximate delays of from zero to severalminutes duration. As previously explained, its rotation ismadein aseries of steps, the time between each step being determined by theelectronic pulse circuit '24. The condensers 20' used with the switch2l-may stand idle for thenecessary periods without appreciable loss ofcharge; while the currentdrain during the period each is connected tothe current input-unit i2 is small.

"The reason for utilizing a current input unit IZ-ofthe type disclosedand described will now be explained. -As those skilled in theartunderstandymost-processes are non-regenerative. For example, aburning fuel releases a certain number -of"heat units regardless of thetemperature within the fire box. If the supply of fuel is reduced, heatdoes not fiow back into the fuel supply as fuel, It is for the'purposeof simulating such a burner that the current input unit l2, consistingof a high gain amplifier'having considerable negative feedback, isutilized. With this current input unit 1:2, a current will flowinthe-output circuit at a predetermined value practically independentofthe impedance into which it isworking when a fixed potential is appliedto its input terminals I I. When the load intowhich the current inputunit i2 is working is composed-of condensers and resistances, as in themesh analogue circuit 15, ithislflow of ourrent simulatestflow ofiheat,while the voltage in any. portion of the resistance-capacitance meshrepresents temperature. If the. demand goes. to zero,..the-.current.input to the :terminals I4 of the resistance-capacitancenetwork |5i fromthe current input unit 12 willbemade :zero, the current will not'l flowback into the source, the. current input unit I 2,- :tov alterthevoltage in any portion ofstheresistance-capacitance mesh, andaccordingly, will not alter the simulated rate of heat dissipation. Ifdecrease; in demand should ocour, the input current will be reduced tosome lower value in accordancewith the demand. In the arrangementsshown, the complete cur-rent supply system has been designed toapproximate asemi-logarithmic valve characteristic in its action. Thecharacteristic of the valve may be predetermined as desired by suitabledesign of theaslidewire resistance 5, as for example, by winding theresistance on a form having a shape corresponding tothe valvecharacteristic desired.

Figs..3, 4 and 5 illustrate in more or less diagrammatic-manner typicalperformance characteristics of the apparatus. disclosed in Figs. 1 and.2. Fig. 3, previously referred to, illustrates the currentflow into theresistance-capacitance network I5 or process analogue for difierentvalues of. impressed voltage on the .input terminals of the.currentinputunit l2. It is noted that the current value is practicallyindependent of the dynamic impedance of .the process mesh circuit.l5..and that the equivalent valve size may be .chosen as desired bychanging the self-biasing resistor 44.

Fig. 4 illustrates the manner in which the dead timeunit 9 delays thevoltage from the valve slidew'ire, while Fig. 5 illustrates severaldiiiervent calibration curves obtainable with the voltmeter l8 by meansof' which the equivalent of suppressed range studies, for example, maybe made.

.Although it is possible, by means of the method and apparatus shown in.the drawings andhere- .in'described, to approximate a particularproczesslthrough regulation of "its resistance and ca- :pacity. itisionly necessary'in practice to arrange certain combinations ofcapacity, transfer lag and dead time to represent whatever particularcharacteristics are desired. In this manner, the effect of variouscharacteristics in automatic control may be investigated by means of theapparatus described Without actually attempting to set up specificprocesses' A principal application for the arrangement disclosed,therefore, .is to simulate process. characteristics rather than toduplicate a specific physical process, although the latter may also bedone if so desired, Examples of types of problems which may be solved inaccordance with the present invention are given hereinafter.

"One typical use to which the invention may be put is the'determinationof the reaction curve of aprocess. A reaction curve, an example of whichis shown in Fig. 6, shows the dynamic action of the process to a suddenchange of control agent input and is obtained by fixing the position ofthe control valve and allowing the measured variable to line out at avalue slightly below the control point. During this perior the processis under'manual control rather than automatic control.

In obtaining a reaction curve of a process, for example, that shown inFig. 6, a small sudden movement of the valve to a new fixed position ismade. If the process under investigation has self regulation, themeasured variable will gradually rise to a new balanced value. If themagnitude of the change has been properly selected, thefinal valueshould be slightly above the control point. The resulting change withtime of the measured variable is the process reaction curve. It is notedthat this reaction curve in cludes the effect of measuring andcontroller lags as well as the process characteristics.

Another use to which the invention may be put is the study of therelative merits of suppressed range controllers. Controllers with asuppressed range scale have come increasingly'lnto use for the purposesof more accurate readability and control of the magnitude of thecontrolled variable. For example, if it is desired to control atemperature of 250 F. it is becoming common practice to use a controllercalibrated to 300 F. instead of 0 to 300F. This suppressed range has aneffect on the controller adjustments because of the different rate ofcontroller pen motion for the same rate of temperature change.

In conducting an investigation of the effect of suppressed rangecontroller scales upon optimum controller adjustments, the first step isto select a suitable process which approximates an actual process of thetype upon which controllers provided with controller adjustments areemployed. Such a controller might, for example, be a proportional resetcontroller. This type of process generally involves slight transfer lagwith moderate process capacity. For the purposes of this investigation,dead time is not required and is accordingly omitted. "Do this end, aswitch 54 isprovided for directly connecting the sliding contact 4 tothe control grid of the triode it and for shunting out the dead timeunit 9.

A representative process, which may be employed for investigating theelfect of a suppressed range-controller scale upon controlleradjustment, is illustrated in the graph shown in Fig. 6 which representsthe reaction curve of a multiple capacity process simulated by means ofthree resistance-ca-pacitance networks, as shown in Fig. 6. The valuesof resistance 41 and condenser 48 in each network except the first,namely that connected to the input terminals M, are respectively 300,000ohms and 300 microfarads. The values of the resistance 41 and condenser48 connected across the input terminals M are respectively 120,000 ohmsand 750 microfarads. In each case the resistance 4'l includes theleakage resistance of its associated condenser. Eachresistancecapacitance network is connected by a resistance 46 of a valueof 40,000 ohms to cause an equivalent temperature drop between thecondensers. In order to obtain a fixed rate of decrease in the variablebeing measured, the condensers are shunted by high value resistanceswhich approximate a loss of heat to atmosphere from each condenser. Theresistance across the last condenser is for convenience called thedemand. Its value determines the heat flow required from the demandcapacity.

The next step in the investigation is to determine the valve size, whichis accomplished by setting the range adjuster on the current input unit,namely the resistance 44, so that the full open position of the controlvalve provides the same rate of rise in the variable being measured asrate of fall when the valve is completely closed. In the instantinvestigation, the proper valve range was determined to be to 100microamperes, corresponding to the lowest and highest controller outputpressures. The next step is to check the valve size to insure that a 50%to 75% setting of the control valve maintains the variable beingmeasured at the control point. Any adjustments which may be required maybe made by varying the magnitude of resistance 44.

A potentiometer pneumatic controller, as slrown at l in Figs. 1 and 2,may be connected to the process previously described, and adjustments ofproportional band (throttling range) and reset rate selected forcontrol. of the process. The adjustments are based on minimum arearecovery for a supply change. Proportional band is defined as the rangeof values of the controlled variable which corresponds to the fulloperating range of the final control element or valve. Reset rate is thenumber of times per minute that the effect of the proportional actionupon the final control element or valve is duplicated by thepnoportional speed floating action, which, in turn, is defined as acontroller action in which there is a continuous linear relation betweenvalues of the controlled variable and rate of motion of the finalcontrol element.

The supply change may be manually introduced by means of a switch 56,battery 5?, slidewire re sistance 50 and contact 59 provided inassociation with the slidewire resistance 5 so that a sudden additionalbias voltage is inserted in the sliding contact lead on the controlvalve slidewirc. For example, the supply change may be of such magnitudethat when the voltage is introduced, a 25% increase in flow is causedwith the actual valve position remaining unchanged. This corresponds toa change in upstream pressure at the control valve, causes a greaterflow of control agent, and requires that the controller correct for thischange.

The potentiometer controller I may then be calibrated to various spansand the suppression adjusted so that the control point represents apredetermined value: for example, three volts. The controlleradjustments may then be redetermined. The following table shows theresults obtained for the example being considered, while in Fig. 7 areshown the typical recovery curves for each condition.

Controller Proportional 355 Max. De- Calibration, Band-s, Per viation,Span-Volts Percent Scale Minute Volts From a theoretical standpoint, itwould be expected that the proportional band would follow a, reciprocallaw such that as the controller span is reduced by one-half, theproportional band would be doubled. The results of these tests are inapproximate agreement as may be seen from the above table. Theproportional band is smaller than expected, however, at the smallerscale spans having the greater amount of suppression.

This discrepancy is capable of explanation as follows. Ifnon-linearities exist in the control system, it is possible that thegreater deviation in percent of scale, representing the same deviationin voltage for various spans, may require a smaller proportional band inorder to achieve the same stability in the recovery curve. Since thecontroller is calibrated for linear response of voltage input to airpressure output, it is unlikely that this is the cause of thediscrepancy.

A second cause may be the difference in apparent dead zone of thecontrol system with respect to changes in voltage of the process whenvarious controller spans are utilized. For example, a dead zone of 0.04%of controller scale with a six volt span would represent 0.0024 volt,while at a two volt span the dead zone would represent 0.0008 volt.Since the process possesses slight transfer lag, the decrease inapparent dead zone with smaller spans would allow a slightly smallerproportional band, thus at least partially accounting for thediscrepancy.

The third cause may be that controller adjustments were not set to theoptimum values to produce a recovery curve having the smallest possibleperiod. This may further account for the discrepancy. In act, therecovery curves of Fig. 7 indicate that the period increases slightly asthe span is decreased. Controller adjustments were made in each casewithout changing the reset rate from the previous setting. It was foundnecessary in every test to decrease the reset rate in order to achievethe desired stability. Although the reset rate should be unchanged withvarious spans, the required decrease in reset rate may be in part due tothe method of selecting controller adjustments.

It is worthwhile to note that in using any type of analogue forinvestigations in automatic control it is necessary to avoid imposingany limitations on the operation of the analogue under dynamicconditions. If the control valve is required to move to its limit oftravel, if the process reaches a potential beyond which it can not go,or if the controller pen should attain full scale, the complete systemis unable to follow adequately its own laws of operation. Such action inthe system correspondingly influences the dynamic operation unlesslimits are required as in two position control.

From the foregoing it is evident that the electrical analogy method andapparatus herein de- 1 5 scribed for simulating processes-lends itselfreadily to the selection of varying degrees of process characteristics.It has been shown how such factors as process capacity, transfer lag,dead time, valve size, controller scale range, and load changes are setup on the analogue.

Various investigations of automatic control, particularly when actualcontrol equipment is employed, emphasize the necessity of taking intoaccount such factors as measuring lag, controller lag, and dead zones inthe measuring and controlling means, as well as .in the process. Theresults of the investigation of controller span described emphasize thefact that these factors can be neglected only in exceptional cases.Empirical methods of analysis of automatic control must generally betempered by the influence of factors difficult to account for inmathematical methods. These factors which are difficult to account forin mathematical methods may be readily taken into account by means ofthe method and apparatus of the present invention.

Subject matter disclosed in this application and not claimed herein isdisclosed and is being claimed in the .copending application of WilliamH. Wannamaker, Jr., bearing Serial No. 585,125 and filed on even dateherewith, in the copending application of William H. Wannamaker, Jr.,bearing Serial No. 769,555, .filed August 19, 1947, and in the copendingapplications of William H. Wannamaker, Jr.,.bearing Serial Nos. 43,741and 43,742, filed on August '11, 1948. Said application Serial No.585,125 issued as Patent No. 2,453,053 on November 2, 1948.

While, in accordance'with the provisions of the statutes, we haveillustrated and described preferred embodiments of the presentinvention, those skilled in the art will understand that changes may bemade in theform of the apparatus disclosed without departing from thespirit of our invention as set forth in the appended claims, and thatsome features of the present invention may sometimes be used toadvantage without a corresponding use of other features.

Having now described our invention, What we claim as new and desire tosecure by Letters Patent, is as follows:

1. The method of simulating a control system comprising the steps .ofproducing an electrical current having a substantially constantamplitude irrespective of the'impedance through which it flows, passingsaid current through an impedance having electrical characteristicsanalogous to the characteristics of the control system being simulatedto produce a voltage of magnitude jointly determined by the amplitudeofsaid current and by said electrical characteristics, and varying theamplitude of said current as required to restore said voltage to apredetermined value upon departure of said voltage'from said value.

2. The method of simulating a control system comprising the steps ofproducing an electrical current having a substantially constantamplitude irrespective of the impedance through which. it flows, passingsaid current through an impedance having electrical characteristicsanalosons to the characteristics of the control system being simulatedto'zproduce a voltage of magnitude jointly determined by the amplitudeof said current and by said electrical characteristics, and varying theamplitude of said current in accordance with the variations in saidvoltage tomain- -tain said voltage at a substantially constant value.

3. The methodcf simulating ,a control system comprising the steps ofproducing an electrical current having .a substantially constantamplitudeirrespective .of the impedance through which it flows, passingsaid current through an impedance having electrical characteristicsanalogous to the characteristics of the control system being simulatedto produce a voltage of magnitude jointly determined by the amplitude ofsaid current and by said electrical characteristics, measuring saidvoltage, producing a control potential of magnitudedetermined by themagnitude of said voltage, and applying said control potential. to varythe amplitude of said current in accordance with the variations inmagnitude of said control potential.

4. The method of simulating a control system comprising the steps ofproducing an electrical current having a substantially constantamplitude irrespective of the impedance through which it flows, passingsaid current through an impedance exhibiting electrical characteristicscorresponding to certain characteristics of the control system beingsimulated to produce a voltage of magnitude jointly determined by theamplitude of said current and by said electrical characteristics,measuring said voltage, producing a control potential of magnitudedetermined by the magnitude of said voltage, and applying said controlpotential to vary the amplitude of said current in accordance with thevariations in magnitude of said control potential in such manner thatchanges insaid control potential are operative to vary the amplitude ofsaid current only after the lapse of a time period corresponding toanother characteristic of the control system being simulated.

5. The method of simulating a control system comprising the steps ofproducing an electrical current having a substantially constantamplitude irrespective of the impedance through which it flows,producing a voltage of magnitude ultimately corresponding to themagnitude of said current but being characterized in that changes insaid current are reflected as changes in said voltage only after aretardation period corresponding to capacity and transfer lag in thecontrol system being simulated, measuring said voltage, producing acontrol potential of magnitude determined by the magnitude of saidvoltage, and applying said control potential to vary the amplitude ofsaid current in accordance with the variations in magnitude of saidpotential.

6. The method of simulating a control system comprising the steps ofproducing an electrical current having a substantially constantamplitude irrespective of the impedance through which it flows,producing a voltage of magnitude ultimately corresponding to themagnitude of said current but being characterized in that changes insaid current are reflected as changes in said voltage only after aretardation period corresponding to capacity and transfer lag in thecontrol system being simulated, measuring said voltage, producing acontrol potential of magnitude determined by the magnitude of saidvoltage, and applying said control potential to vary the amplitude ofsaid current in accordance with the variations in magnitude of saidcontrol potential in such manner that changes in said control po tentialare operative to vary the amplitude of said current only after the lapseof a time period corresponding to the dead time of the control system.beingsimulated.

7. Apparatus for simulating a control system comprising a deviceoperative to produce an electrical current having a substantiallyconstant amplitude irrespective of the impedance through which saidcurrent flows, an impedance exhibiting characteristics analogous tocertain characteristics of the control system being simulated, means topass said current through said impedance to produce a voltage dropacross at least a portion of said impedance, said voltage drop having amagnitude jointly determined by the amplitude of said current and by thecharacteristics of said impedance, and means responsive to the magnitudeof said voltage drop to vary the amplitude of said current as requiredto maintain said voltage drop substantially at a predetermined value.

8. Apparatus for simulating a control system comprising a deviceoperative to produce a variable electrical current having asubstantially constant amplitude irrespective of the impedance throughwhich said current flows, means to adjust to a predetermined value therange of variation of said current, an impedance exhibitingcharacteristics analogous to certain characteristics of the controlsystem being simulated, means to pass said current through saidimpedance to produce a voltage drop across at least a portion of saidimpedance, said voltage drop having a magnitude jointly determined bythe amplitude of said current and by the characteristics of saidimpedance, and means responsive to the magnitude of said voltage drop tovary to a value within said range the amplitude of said current asrequired to maintain said voltage drop substan tially at a predeterminedvalue.

9. Apparatus for simulating a control system comprising a deviceoperative to produce an electrical current having a substantiallyconstant amplitude irrespective of the impedance through which saidcurrent flows, an impedance exhibiting characteristics analogous tocertain characteristics of the control system being simulated. means topass said current through said impedance to produce across at least aportion of said impedance a voltage drop having a magnitude jointlydetermined by the amplitude of said current and by the characteristicsof said impedance, means to measure said voltage drop and to produce acontrol potential of magnitude varying in accordance with the magnitudeof said voltage drop, and means operative after the lapse of a timeperiod analogous to another characteristic of the control system beingsimulated to vary the amplitude of said current in accordance with thevariations of magnitude of said control potenial.

10. Apparatus for simulating a control system comprising a deviceoperative to produce an electrical current having a substantiallyconstant amplitude irrespective of the impedance through which saidcurrent flows, an impedance exhibiting characteristicsanalogous tocertain characteristics of the control system being simulated, means topass said current through said impedance to produce across at least aportion of said impedance a voltage drop having a magnitude jointlydetermined by the amplitude of said current and by the characteristicsof said impedance, means to measure said voltage drop and to produce acontrol potential of magnitude varying in accordance with the magnitudeof said voltage drop, means operative after the lapse of a time periodanalogous to another characteristic of the control system beingsimulated to. vary the amplitude of said current in accordance with 11.Apparatus for simulating a control system comprisin a device operativeto produce a variable electrical current having a substantially constantamplitude irrespective of the impedance through which said currentflows, means to adjust to a predetermined value the range of variationof said current, an impedance exhibiting characteristics analogous tocertain characteristics of the control system being simulated, means topass said current through said impedance to produce across at least aportion of said impedance a voltage drop having a magnitude determinedjointly by the amplitude of said current and by the characteristics ofsaid impedance, means responsive to the magnitude of said voltage dropand operative after the lapse of a time period analogous to anothercharacteristic of the control system bein simulated to vary to a valuewithin said range the amplitude of said current as required to maintainsaid voltage drop substantially at a predetermined value, and means tovary the length of said period.

12. Apparatus for simulating a control system comprising a deviceoperative to produce a vari able electrical current having asubstantially constant amplitude irrespective of the impedance throughwhich said current flows, means to adjust to a predetermined value therange of variation of said current, an impedance exhibitingcharacteristics analogous to certain characteristics of the controlsystem being simulated, means to pass said current through saidimpedance to produce across at least a portion of'said impedance avoltage drop havin a magnitude jointly determined by the amplitude ofsaid current and by the characteristics of said impedance, means tomeasure said voltage drop and to produce a control potential ofmagnitude varying in accordance with the magnitude of said voltage drop,means operative after the lapse of a time period analogous to anothercharacteristic of the control system being simulated to vary theamplitude of said current to a value within said range in accordancewith the variations in magnitude of said control potential, and means tovary the length of said period.

13. Apparatus for simulating a control system comprising a deviceoperative to produce a variable electrical current havin a substantiallyconstant amplitude irrespective of the impedance through which saidcurrent flows, means to gradually vary the value of said electricalcurrent, means to suddenly vary the value of said electrical current,means to adjust to a predetermined value the range of variation of saidcurrent, an impedance exhibiting characteristics analogous to certaincharacteristics of the control system being simulated, means to passsaid current through said impedance to produce across at least a portionof said impedance a voltage drop having a magnitude jointly determinedby the amplitude of said current and by the characteristics of saidimpedance, and means responsive to the magnitude of said voltage dropand operative following actuation of said suddenly or gradually varyingmeans to vary the amplitude of said current to a value within said rangeto maintain said voltage drop substantially at a predetermined value.

14. Apparatus for simulating a control system comprising a deviceoperative to produce a variable electrical current having asubstantially constant amplitude irrespective of the impedance throughwhichsaid current flows, means to graduallyvarythe value of .saidelectrical current, means to suddenlyvary the value of said electriocurrent, means toadjust to a predetermined value-the range of variationof said current, an impedance exhibiting characteristics analogous tocertain characteristics of the control system beingsimulated, means topass said current through said impedance-to produce across at leasta-portion:of -saidimpedancea voltage drop havin ;'-a=magnitude-jointlydetermined by the amplitude=of-said current and by the characteristicsof said impedance, means responsive to the magnitude of'said voltagedrop and operative following the actuation of said suddenly or graduallyvaryingsmeans after the lapse of a timeperiod analogoustoanothercharacteristic of the control system-being simulatedto-vary to a valueWithin said' rangetheamplitude of said current as required-to-maintainsaid voltage drop substantially at apredetermined value-and means tovary,.the length of said period.-

15. Apparatus for-simulating a-control system comprising-adevice'operative to produce an electrical; current having a;substantially l constant amplitudeiirrespective of the impedance throughwhich saidcurrent flows,- ant-impedance exhibiting. electricalcharacteristics analogous to certain characteristics of thecontrolsystem being simulated-, -means-to-pass saidcurrent through saidimpedance to producea voltage drop" across at leastaportionor saidimpedance, and means operativeaiter the lapseof a-time period analogousto-anothencharacteristicof the control system beingirsiinulated to var-ythe magnitude of said voltage.

16.- Themethodof simulating: a control system comprising the steps ofproducing an electrical current having a'- substantially constantamplitude-irrespective of the impedance through which it-flows,'1--.passing said -curren'tthrough an impedanoei'havings electricalcharacteristics analagous to the icharacteristics of the control system:

beingesimulatedto produce a-voltage of magnitude jointly determined bythe amplitude of said current and by said electrical characteristics,-and varying the amplitude of said current as required to-imaintain saidvoltage substantially at a predetermined valuer 1-7. The'method ofsimulating a control system comprising the steps of producing anelectrical current having asubstantiallyconstant amplitude irrespectiveof theimpedancethrough which it flows passing said current through animpedance having electrical characteristics analogoustothecharacteristics of the controlsystembeing simulated' to produce avoltage of-magnitude jointly determined by the amplitude of-- saidcurrent and by said electrical characteristics, and after the lapse of atime period' corresponding to another characteristic ofthe-control-system being simulated-varying the amplitude of' Isaidcurrent as requiredto maintain said voltage at asubstantially'constant value.

18-. The -method of simulating-a control system comprising: the steps of-producing an electrical current having: a substantially constantamplitudeirrespective-of the impedance through which it flows:passing-said current through arr impedance having electricalcharacteristics analogous trrthe characteristicsotthe control-systembeing simulated to produce a voltage of magnitude jointly determined bytheamplitude of said ourrent-and by said electrical characteristics,measurlng said voltage, producing a control potential 20 of magnitudedetermined by the magnitude of said voltage, and applying said controlpotential to vary the amplitude of said current as required to maintainsaid voltage substantially at a predetennin'ed value.

191 The method of simulating a control system comprising the steps'ofproducing an electrical current having 'a' substantially constantamplitude irrespective of the impedance through which it flows, passingsaid current through an impedance exhibitingelectrical characteristicscorresponding to'certain characteristics of the control system'being.simulated to produce a voltage of magnitude jointly determined by'theamplitude of said current'and'by said electrical characteristics,measuring said voltage, producing a control potential of magnitudedetermined by'the magnitude of'said voltage, and after the lapseof'a'time period corresponding toanother characteristic of thecontrol'system being simulated applying said control potential tovary'the amplitudeof said current as required to maintain said voltage"substantially at a" predetermined value.

20; The method of simulating a control system co1nprisin'g the'steps ofproducing an electrical currenthaving a substantially constant amplitudeirrespective of the impedance through which it"fiows; producinga'voltage of magnitude ultimately corresponding to the magnitude of saidcurrent but being characterized inthat changes in said current arereflected as changes in said voltage only after a retardation" periodcorresponding to capacity and transfer lag in the control' system beingsimulated, measuring said volt age, producing a control potential ofmagnitude determined by the magnitude of said voltage, and applying saidcontrol potential to vary the amplitude of saldcurrent as required tomaintain said 'voltage at a'substantially constant value.

21'. The methodof simulating alcontrol system comprising the "steps ofproducing an electrical current having a'substa'ntially constantamplitude irrespective of "the impedance" through which it flows,producing. a voltage of magnitude ultimately corresponding to themagnitude of said current but being characterizedin that changes insaid-current are reflected as changes in said voltage only afteraretardation-period correspond-' ing' to capacity and transfer lag inthe control system being simulated, measuring said voltage, producingacontrol potential of magnitude determined by the magnitude of saidvoltage, and after thelapse of a time period corresponding to the deadtime of the control system being simulated applying said controlpotential to vary theamplitude of said current as required to maintainsaid voltage substantially at a predetermined value.

DONALD P. ECKNIAN. WILLIAMH; WANNAMAKER, JR.-

REFERENCES CITED he -following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date- 2.'0'75,966 Vance Apr. 6, 19372087567 Hedin- July 20, 1937 2.244.369 Martin June 3, 1941 2,263.01!Sparrow Nov. 18, 1941 2.420.391 McCann May 20,1947 2423,754- Bruce- July8, 1947 2,439,891 Hornfeck- Apr, 20, 1948

